This invention relates to the art of electrode manufacture. More specifically, it relates to the art of manufacturing fluoride sensitive electrodes employing a fluoride sensitive membrane or crystal.
Numerous electrodes have been developed in the art which are capable of ion selectivity. U.S. Pat. No. 3,431,182 discloses an electrode suitable for measuring fluoride ion activity in an aqueous solution. The electrode of the patent employs a fluoride ion sensitive membrane of a solid ionic fluoride compound which membrane is substantially insoluble in the aqueous fluoride composition. The membrane is contacted at one surface by the aqueous composition and at the other surface by a reference electrolyte providing a fixed contact potential. Exemplary membranes are formed of fluorides of the lanthanide series of selected rare earth metals.
While the electrode design of the above patent is effective to measure the fluoride ion activity in a process solution, it has been found that the service life of such electrodes is highly unsatisfactory. Most of such electrodes have a service life of less than 6 months and, in some cases, as short as 2-3 weeks. The problem which has been experienced is a loss of seal between the sensing membrane and the plastic body of the electrode shell. The show-setting cement employed to adhere the membrane to the shell begins to crack and permits the process fluid to enter the internals of the electrode. Epoxy cements are normally employed to secure the membrane to the shell. Epoxy cements generally exhibit resistance to chemicals and are capable of withstanding modest temperature variations; however, the epoxy cement is apparently unsuitable for this particular application in measuring the fluoride content of the process solutions.